In a large mixing bowl, beat together butter and sugars until smooth and creamy. Mix in eggs, one at a time, beating until well combined after each. Beat in vanilla. Add dry ingredients and mix just until combined. Stir in bittersweet chocolate.

Scoop mixture into a large 12" x 18" rimmed baking sheet coated with cooking spray. Using an off-set spatula or dampened fingertips, evenly spread dough to cover the bottom of the sheet. Place pan into the oven and bake, rotating sheet halfway through, until the edges are brown and the top is golden, about 38 to 45 minutes.

Remove from the oven and place pan on a wire rack to cool completely before slicing into bars.